Cathodic protection method



United States Patent Olfice 3,537,963 Patented Nov. 3, 1970 3,537,963 CATHODIC PROTECTION METHOD Charles F. Schrieber, Lake Jackson, Tex., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Continuation-impart of application Ser. No. 535,002, Mar. 17, 1966. This application Apr. 10, 1969, Ser. No. 815,222

Int. Cl. C23f 13/00 U.S. Cl. 204148 6 Claims ABSTRACT OF THE DISCLOSURE The invention comprises a cathodic protection system, to protect a metal from corrosion, comprising an anode in direct contact with a sulfide-containing environment and in electrical contact with the metal to be protected; the anode consisting essentially of 0.01 to 0.3 weight percent Hg and at least 2 weight percent Zn, the balance being aluminum.

This application is a continuation in part of Ser. No. 535,002, filed Mar. 17, 1966, now abandoned.

BACKGROUND OF THE INVENTION The present invention relates to the cathodic protection of metallic structures, such as pipelines or tanks with sacrificial anodes, in direct contact with a sulfidecontaining envoronment, e.g. the ocean floor, saline mud, or any bed of a body of water.

Conventionally, zinc is used as the galvanic sacrifical anode for protection of such structures. However Zinc has a relatively low ampere hour capacity, i.e. current efficiency, of about 370 ampere-hours per pound (amphrs./lb.).

Reding (U.S. 3,321,306) discloses an aluminum base sacrificial anode containing 0.002 to 0.2 weight percent mercury and 0.1 to 1 weight percent zinc for use particularly in a sea water environment. But, although the curent efiiciency of this anode is relatively high in sea water (about 1250 amp-hrs./lb.), its efiiciency drops to only about 560 amp-hrs./lb. in a sulfide environment.

It is a principal object of the present invention to provide a cathodic protection system for metals wherein the sacrificial anode is in contact with a sulfide environment.

It is a further object of the present invention to provide a method of improving the current efficiency of an Al-Hg-Zn anode in a sulfide environment.

THE INVENTION These and other objects and advantages are found in cathodic protection system for metal structures which comprises an anode in direct contact with a sulfide-containing environment and electrically connected to the metal to be protected, said anode consisting essentially of from about 0.01 to about 0.3 weight percent Hg and from about 2 to about 20 weight percent Zn. Preferably the zinc is in a range of from about 2.5 to 15 weight percent.

Such system better protects the metal structures due to improved anode current efficiency.

As mentioned, Al-Hg anodes with low Zinc, e.g. less than 1 weight percent, have a low current efliciency (560 amp-hrs./lb.) in sulfide environment applications. By using the system of the present invention, the current efficiency is nearly doubled, eg 950 to 1150 amp-hrs./lb. Such improvement is noted with respect to aluminum based anodes containing 0.01 to 0.3 weight percent mercury.

Apparently this improvement results from the formation of 2113 which protects the aluminum anode from attack by the environmental sulfides, e.g. H 5.

The mercury is preferably maintained within the range of from about 0.02 to about 0.08 weight percent. Anodes having more than about 0.08 weight percent tend to oxidize when in contact With air, e.g. during storage.

The anode is electrically connected to the protected metal by techniques known to those skilled in the art, such as by welding, clamping or bolting the anode to the metal.

I have discovered that the current efliciency of an aluminum base sacrificial anode containing mercury and zinc, e.g. the Reding anode, in a sulfide-containing environment can be improved by increasing the zinc content thereof to at least 2 weight percent. The zinc content can be increased to as high as 20 weight percent. Preferably the Zinc is maintained within the range of 2.5 to 15 weight percent.

A further embodiment of the present invention is a method of improving the current efficiency of an Al-Hg- Zn anode used in a sulfide soil environment by increasing the zinc content to at least 2 weight percent.

In practicing the method of the present invention, conventional alloying and casting or fabricating techniques, as practiced by those skilled in the art, can be employed. The alloying constituents can contain those amounts and types of impurities normally found therein.

The following examples will serve to further illustrate the present invention but are not meant to limit it thereto.

Examples 1-6 Cylindrical anode specimens about 5 /2 inches long and about /3 inch in diameter were placed in direct contact with sea water saturated mud containing soluble sulfide in one-half gallon glass jars. A steel wire mesh was placed adjacent to the inner wall of each jar as a cathode. The cells were completed with respect to electrical circuitry, a rectifier being employed to maintain a constant current through a group of cells in series. The cells were run for a number of hours at a constant current density of milliamperes per square foot and the current efficiency measured. The results of such tests are shown in Table I.

TABLE I.LABORATO RY TESTS Composition 1 Current;

Hg wt., Zn wt., Days elliciency,

Example percent percent on test amp-lns/ll) Comparative-A 0. 025 0. 34 148 475 Comparative-B 0. 046 0. 54 750 Comparative-C. 0. 06 0. 49 45 560 Comparative-D 0. 05 0. 5 52 640 Comparative-E 0. 06 l. 0 96 560 1 0. 06 3. O 90 070 2- 0. l2 4. 1 148 950 3- r 0. 07 4. 5 148 1,020 4 0.06 4.2 45 970 5 0.05 5.0 52 950 6 0.20 11. 2 134 800 1 Balance being A1 (99.9% purity).

The current efficiency of Comparative Examples A-E while quite high, i.e. 1250 amp-hrs./lb., in sea water (see Reding patent referred to above), is drastically reduced in a sulfide environment.

The system of the present invention, as exemplified by Examples 1-6, the current efficiency is substantially enhanced to nearly 1000 amp-hrs./lb., thereby affording increased protection.

The procedure of Examples 1-6 was repeated using sea water saturated with H 8 as the sulfide environment. An anode of Comparative Example D composition had a current efiiciently of 930 amp-hrs./lb. or 70% of the theoretical value. However, an anode of Example 5 com- 3 position had a currently efficiency of 1200 amp-hrs./lb.

or 94% of the theoretical value.

Examples 714 This improvement is even more dramatically presented in the field test examples. Weighed anodes with a steel core inserted into them were placed in the mud at the bottom of a seat water canal. The anodes dimensions were either 1.4" x 6" x 11" or 2" x 2" x 30". A lead wire was silver soldered onto each anode core and a fixed known resistor was inserted into the wire connecting the anode to the cathode. The cathode was an underground pipeline which crossed the sea water canal. Once every two weeks the voltage drop across the fixed resistor, the cathode potential, and the circuit resistance were measured. The anode current output could be calculated by dividing the voltage across the fixed resistor by the resistance of the fixed resistor. (The anode potential could be calculated by multiplying the anode current output by the circuit resistance and then adding the product to the cathode potential.) At the conclusion of the test, the anodes were removed, cleaned by wire brushing and by immersing in concentrated HNO at room temperature, dried, and weighed. The total current delivered by each anode was then calculated by integrating the current versus time curve. The anode eificiency was then calculated by dividing the total current delivered (amp-hrs.) by the anode weight loss (lb).

The results are shown in Table II.

TABLE IL-FIELD TESTS Composition 1 Current Hg, wt. Zn, wt. Days effieiency,

Example percent percent on test amp-hrs./lb

Comparative-F 0. 038 0. 38 370 450 Comparative-G 0. 033 0. 47 370 790 Comparative-IL 0. 056 0. 62 370 580 Comparative-J. 0. 042 0. 69 212 380 7 0. 028 2. 8 132 1, 220 0. 045 3. 1 132 1, 300

1 Balance being Al (99.9% purity).

4 rent efficiency. The system of the present invention has double the current efficiency to produce a far superior corrosion protection.

The system can thus be used to effectively protect pipelines and other metal structures where in the anode is in direct contact with a sulfide-containing environment, such as the ocean floor, lake bed or river bottom, or aqueous environment containing sulfides.

What is claimed is:

1. A method of cathodically protecting a metal from corrosion, comprising disposing an anode in direct contact with a sulfide-containing environment selected from the group consisting of the bed of a body of water or a water environment essentially saturated with sulfide and electrically connecting said anode to the metal to be protected, said anode consisting essentially of from about 0.01 to about 0.3 weight percent Hg and from about 2.0 to about 20 weight percent Zn, the balance being aluminum.

2. The method of claim 1 wherein in the anode, the zinc is present in an amount within the range of from about 2.5 to about 15 weight percent.

3. The method of claim 1 wherein in the anode, the mercury is present in an amount within the range of about 0.02 to about 008 Weight percent.

4. The method of claim 1 wherein in the anode the zinc is present in an amount within the range of from about 2.5 to about 15 weight percent and the mercury is an amount within the range of from about 0.02 to about 0.08 weight percent.

5. The method of claim 1 wherein the environment is the bed of a body of water.

6. The method of claim 1 wherein the environment is an ocean floor.

References Cited UNITED STATES PATENTS 3,281,239 10/1966 Reding et a1. 75-138 3,321,306 5/1967 Reding et a1. 204148 3,343,948 9/1967 Raclot '75l38 TA-HSUN G TUN G, Primary Examiner U.S. Cl. X.R. 

